The use of fast Internet connections has grown rapidly over the last few years, and consequently the demand for broadband (high-speed) connections is increasing.
One technology that is very well known in the market is Asymmetric Digital Subscriber Line (ADSL) technology. This employs the frequency spectrum indicated schematically in FIG. 1. “Upstream” communications (that is in the direction from the home or office user premises, “customer premises equipment” or “CPE”, to the “central office”, or “CO” or DSLAM, FTTC, or Fibre To The Curb, or FTTH, Fibre To The Home cabinets) are transmitted on frequencies in the range of 25 kHz (i.e., above the maximum audible frequency of 4 kHz) to 138 kHz. “Downstream” communications are in a higher frequency band from 138 kHz to an upper limit. According to the first two versions of ADSL (ADSL and ADSL2) the downstream band goes up to 1.1 MHz, whereas in ADSL2+ it goes up to 2.2 MHz. The upstream can be also extended from 0 khz up to 276 kHz, also known as All Digital Loop and extended upstream. Within each of the upstream and downstream bands, the range is divided into 4 kHz intervals, “tones,” so that the downstream band includes 256 tones in ADSL and ADSL2 (which is capable of transmitting 8 MBps), and 512 tones in ADSL2+ (which is capable of transmitting 28 MBps). Each tone is encoded by quadrature amplitude modulation (“QAM”), and can encode between 0 and 15 bits. During a training phase, the line conditions (signal to noise ratio, SNR) of each of the tones is estimated, and the number of bits which will be encoded in each tone during each frame is selected.
In a typical ADSL modem, the main sections are (i) a Digital Interface (which may use asynchronous transfer mode (ATM)); (ii) a Framer (also referred to here as a framing unit); (iii) an DMT Modulator; (iv) the AFE (Analog Front End); and (v) a Line Driver.
The framer multiplexes serial data into frames, generates FEC (forward error correction), and interleaves data. FEC and data interleaving corrects for burst errors. This allows DMT-based ADSL technology to be suitable for support of MPEG-2 and other digital video compression techniques. For the transmit signal, an Encoder encodes frames to produce the constellation data for the DMT Modulator. It assigns the maximum number of bits per tone (based on measured SNR of each tone) and generates a QAM constellation where each point represents a digital value. Each constellation point is one of N complex numbers, x+iy, where x and y are the phase and amplitude components. The summation of bits in all carriers, multiplied by the frame rate (4 kHz), represents the data rate. For the receive signal, the decoder converts QAM symbols back into the data bitstream.
In the DMT Modulator, a frequency domain processor implements FFT/IFFT and associated processing. In the transmit path, the Inverse Fast Fourier Transform (IFFT) module accepts input as a vector of N QAM constellation points and duplicates each carrier with its conjugate counterpart so the 2N output samples are real. The 2N time domain samples may have for example the last 2N/16 samples appended as a cyclic extension (which may include a cyclic suffix, a windowing function and/or a cyclic prefix extension) for every symbol, and are then delivered to a DAC (digital-to-analog converter). The set of time domain samples represents a summation of all the modulated sub-channels, for the duration of one data frame. In the receive path, the first 2N/16 samples (cyclic prefix) from the ADC are removed from every symbol. A FFT module transforms the carriers back to phase and amplitude information (N complex QAM symbols). Correction for attenuation of the signal amplitude and phase shifts (i.e., overall distortion) is implemented. If the QAM constellation is thought of as points in a grid where rows and columns represent phase and amplitude information respectively, then the grid effectively rotates reference to the constellation points to correct for these distortions.
Based on the SNR, which has been established for the tones, they are classified based on the SNR such that a “path” is selected for each tone through the encoding device, and each of the tones is transmitted along to the framing unit through the corresponding selected transmission path. This is illustrated in FIG. 2(a), in which the framing unit 1 for producing V/ADSL frames receives data along two paths 2, 3. Each path 2, 3 leads to a respective block 4, 5, which constructs respective portions of frames. The frame is shown in FIG. 2(b), including a portion 6 generated by block 8, and a portion 7 constructed by a block 9 (which may be an interleaver). The outputs of the blocks 4, 5 are stored respectively in a fast buffer 8 and interleaved buffer 9, until they are transmitted out of the framing unit 1. Since the interleaver 5 interleaves data over a period of time, data transmitted along path 3 will have a different (higher) latency than data transmitted along the path 2. Thus, these two paths are referred to as different “latency paths” (e.g., they may be referred to as LP1 and LP2). Note that both paths LP1 and LP2 may be interleaved.
DMT technology also includes a feature known as “tone ordering”. This means that the encoder, in forming VDSL symbols (there may be multiple VDSL frames within one VDSL symbol), determines the order in which subcarriers are assigned bits. The term tone ordering is wide enough to include both (i) determining the order in which the subcarriers are assigned data transmitted along a given latency path; and (ii) the order in which the subcarriers are assigned data transmitted along the different latency paths.
Furthermore, the number of bits that are transmitted by each of the tones may be modified if the estimated SNRs of the tones are revised: increasing the number of bits stored per frame in some tones and correspondingly reducing the number of bits stored per frame in other tones. There could be other reasons to dynamically change the bit allocation for spectral reasons too. This process is known as “bit swapping.”
For further details of the ADSL2 standard, the reader is referred to the document ITU-T Recommendation G.992.3 published by the International Telecommunication Union, the disclosure of which is incorporated herein by reference in its entirety.
While ADSL provides Internet connections that are many times faster than a 56K modem, they still are not fast enough to support the integration of home services such as digital television and Video-on-Demand. However, another DSL technology known as very high bit-rate DSL (VDSL) is seen by many as the next step in providing a complete home-communications/entertainment package.
In contrast to ADSL, a conventional VDSL standard (here referred to as VDSL1) uses a number of bands, e.g. as shown in FIG. 3, which may go up to, for example, 12 MHz. Data rates are typically larger than those of ADSL, e.g., 8 k samples per VDSL symbol for 4096 point-FFT. VDSL has a number of further differences from ADSL. For example, VDSL1 has different framing methods from ADSL2 (for example, with no sync symbol), it does not include Trellis encoding, and its interleaving system is different. In the ADSL2 system, the tone ordering is applied to all the tones used for communication in a given direction. Up until now, two sets of memories were required on a chip. If this feature is incorporated into future versions of VDSL, here referred to as VDSL2, with 4 k tones or higher, each of the bit allocation table, gain tables, tone ordering tables each for 4 k tones requires significant on-chip memory.